help needed with sort. pipe and popen() - UNIX for Dummies Questions & Answers

the_learner · #1 05-17-2007

Suppose I want to find the number of uniq lines in a file. I use the following command:

sort file1 | uniq -c | wc -l

But if for some reason sort fails, the above command returns 0 as the answer.

Why would sort fail ? sort makes use of directories /tmp or /var/tmp to store temporary files. So what I did is I renamed the temp dirs. And thats how sort failed..

After renaming the temp dirs:

Quote:

%> sort file1 | uniq -c | wc -l
sort: can't open /var/tmp/stmAAAEVayZ4.00000001: No such file or directory
0

So sort fails, but notice the 0 which is returned as the result.

Now, I have a program and I use popen() to execute the sort | uniq |wc command. What I want is, if sort fails, I should be able to catch it. But my program is not able to do so, because of the 0 returned as the result.

I have:

Code:

FILE *fptr;

fptr = popen("sort file1 | uniq -c | wc -l" , "r");

if (fptr != NULL) {
     fscanf(fptr, "%lu", &uniqLines);
} else {
    //ERROR
}

So even if sort fails, popen() writes a 0 to the stream pointed by fptr and uniqLines is always 0.

Please help ! !! !!!

the_learner · #2 05-17-2007

Sorry, I think this is more of a C programming question. Can you please move it there ?

the_learner · #3 05-17-2007

I guess what I am looking for is: When commands are piped, if any one command fails, the commands that follow should not execute...

porter · #4 05-17-2007

Quote:

Originally Posted by the_learner

I guess what I am looking for is: When commands are piped, if any one command fails, the commands that follow should not execute...

No can do. Commands that are piped are all launched at the same time, so all those to the right have already been started before sort decides it's having a bad day.

You would need to execute the commands one after the other with intermediate files.

the_learner · #5 05-18-2007

Ok it seems that the status of each command in the pipe is available in the array PIPESTATUS.

So echo ${PIPESTATUS[0]} will give me the status of the first command.
echo ${PIPESTATUS[1]} will give me the status of the second command and so on.

BUT now the problem is that PIPESTATUS is only available in bash. Where as popen() opens an sh shell.

And so I cannot do this:

Quote:

fptr = popen("sort file1 | uniq -c | wc -l; echo ${PIPESTATUS[0] > some_file" , "r");

porter · #6 05-19-2007

And what is the problem with doing

Code:

FILE *fp=popen("/usr/bin/bash -c \"..........\"","r");

?

(edit, bash not ksh)

the_learner · #7 05-21-2007

Hey, can you please tell me what does ksh -c do ? (or give some examples)

PIPESTATUS is only availablein bash and not in ksh.

I switched my shell to ksh and then executed:

/usr/bin/bash -c "echo ${PIPESTATUS[*]}" but that did not work.

Thanks porter !

(On 2nd thoughts it might not be a good thing to rely on a specific shell feature, hmm.. I guess I will have to write my own C program to implement PIPES (I am trying but its driving me crazyy, I shall post my queries soon :P )

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